Diode strings and electrostatic discharge protection circuits

ABSTRACT

Diode strings and electrostatic discharge circuits characterized by low current leakage. Each diode region provides a diode and has first and second regions. The first region is of a first conductive type and formed on a substrate, acting as a first electrode of a diode. The second region is of a second conductive type opposite to the first conductive type, formed in the first region and acting as a second electrode of a corresponding diode. The diodes are forward connected in series to form major anode and cathode of the diode string. An isolation region is of the second conductive type to isolate those diode regions. A bias resistor is connected between the isolation region and a first power line. During normal operation, the voltage of the first power line is not within the range between the voltages of the major anode and cathode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent application Ser. No. 11/004,348 filed on Dec. 3, 2004.

BACKGROUND

The present invention relates in general to diode strings and relevant ESD protection circuits. More particularly, it relates to diode strings and ESD protection circuits characterized by low current leakage during normal operation.

Among ESD protection devices, proper forward biasing of a diode during an ESD event allows only a small silicon area to be required for effective ESD protection.

A conventional diode string consists of several diodes connected in series. When coupled between high-voltage and low-voltage power lines, the conventional diode string acts as an ESD protection circuit, clamping the voltage across the power lines and protecting devices from high voltage stress. Nevertheless, a parasitic Darlington amplifier may be formed in the diode string by series-connected parasitic bipolar junction transistors (BJTs), resulting in constant substrate current leakage forward to a substrate. This substrate current leakage becomes more severe as operating temperature or diode count in the diode string increases.

Conventional solutions to the leakage problem include adding extra circuitry to reduce the current gain of the Darlington amplifier, or physically elimination of the Darlington amplifier.

SUMMARY

An object of the present invention is to reduce current leakage of a diode string during normal operation.

Another object of the present invention is to provide a diode string and a relevant ESD protection circuit characterized by low current leakage during normal operation.

A diode string is provided, comprising diode regions, a separation region and a bias resistor. Each diode region provides at least one diode and has first and second polarity regions. The first polarity region is of a first conductivity type on a substrate, acting as a first electrode of a corresponding diode. The second polarity region is of a second conductivity type opposite to the first conductivity type, in the first polarity region, acting as a second electrode of the corresponding diode. The diodes are connected in series to provide a major anode and a major cathode of the diode string. The separation region is of the second conductivity type, separating the diode regions from each other. The bias resistor is connected between the separation region and a first power line. During normal operation the voltage of the first power line is not within the range between the voltages on the major anode and cathode.

Another diode string characterized by low current leakage is further provided, comprising bipolar junction transistors (BJTs) and a bias resistor. Each BJT has a collector, an emitter and a base. The emitter of a BJT is connected to the base of the following BJT, and the base of the first BJT and the emitter of the last BJT act as two major electrodes of the diode string. The collectors of all the BJTs are coupled to a first power line through the bias resistor. During normal operation the voltage of the first power line is not within the range between the voltages of the major electrodes.

The bias resistor decreases the collector current in a parasitic Darlington amplifier, thereby depressing the current leakage during normal operation.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is made to a detailed description to be read in conjunction with the accompanying drawings, in which:

FIG. 1 a is a cross section of a diode string according to an embodiment of the present invention;

FIG. 1 b shows an equivalent circuit for the diode string in FIG. 1 a and a newly-defined symbol for a diode string embodying the present invention;

FIG. 2 shows a circuit with electrostatic discharge (ESD) protection circuits according to the present invention;

FIGS. 3 a and 3 b show two ESD protection circuits, each having an ESD protection trigger circuit according to the invention and connected between power lines VCC and GND; and

FIGS. 4 to 6 show cross sections of three diode strings embodying the present invention.

DETAILED DESCRIPTION

FIG. 1 a is a cross section of a diode string according to an embodiment of the present invention. In FIG. 1 a, a diode string 100 is formed on a P-type substrate 102 of a chip, fabricated by, but not limited to, triple-well CMOS process.

Diode string 100 has diode regions 104, each providing at least one diode and having a P well 106 and a heavily-doped N region 108. P well 106 and heavily-doped N region 108 respectively act as an anode and a cathode of a diode. N well 100 surrounds diode regions 104. Deep N well 112 is deeper than N well 100 and contacts the bottoms of diode regions 104. N well 100 and deep N well 112 act as a separation region to electrically isolate P wells 106 not only from each other but also from P substrate 102.

To form effective electrical contacts, P wells 106 have heavily-doped P regions 114, N well 100 a heavily-doped N region 116, and P substrate 102 a heavily-doped P region 118. The count of these heavily-doped regions is not fixed, depending on how little contact resistance is required by the circuit design. Depending upon the technology of the manufacturing process, heavily-doped regions can optionally be formed with silicide material on their surface to reduce sheet resistance. Isolation material can be formed on the surface of the chip to electrically isolate heavily-doped regions. In the embodiment of FIG. 1 b, the isolation material is formed and positioned by, but not limited to, shallow trench isolation. Other process, such as local oxidation, can also be applied to form and position isolation material.

Interconnection on a chip, generally comprising metal wires, contacts and vias, provides connections for devices. Through interconnection, the diodes are forward connected, whereby a cathode of a diode is connected to an anode of a following diode and the anode of the first diode and the cathode of the last diode respectively act as the major anode and cathode of the diode string 100. The count of the diodes connected in series depends upon the desired threshold voltage of the diode string. If the desired threshold voltage of the diode string is at least 4v and each diode can contribute 0.7v for the threshold voltage, the diode count in the diode string must not be less than 6, since 0.7×6=4.2>4.

Through interconnection, P substrate 102 is coupled to power line GND. Deep N well 112 and N well 100 are connected to a bias resistor Rb and then coupled to power line VCC. Bias resistor Rb can be, but is not limited to, a poly-silicon resistor or a well resistor.

During normal operation, when no ESD occurs and the chip is properly powered, the voltage of power line VCC cannot be less than that on the major anode, to maintain junction reverse biasing of a PN between deep N well 112 and P well 106. In the embodiment of FIG. 1 a, the voltages of power lines VCC and GND can be the highest and lowest voltage on the chip, respectively.

The left portion of FIG. 1 b shows an equivalent circuit for the diode string in FIG. 1 a and the right portion a newly-defined symbol for a diode string embodying the present invention.

Comparing the equivalent circuit of FIG. 1 b with the device structure in FIG. 1 a, the collector of each NPN BJT is deep N well 112 or N well 110, the base a P well 106, and the emitter a heavily-doped N region 108. Due to the forward connection, the emitter of a BJT is connected to the base of the next BJT. The base of the first BJT acts as the major anode of the diode string 100, and the emitter of the last BJT as the major cathode of the diode string 100. All collectors are connected and then coupled to power line VCC through bias resistor Rb.

The connection of the BJTs in FIG. 1 b shows a Darlington amplifier with bias resistor Rb suppressing the collector current through the Darlington amplifier, thereby decreasing current leakage during normal operation. In another aspect, there is no resistor loaded on the current path from the major anode and the major cathode. Therefore, when the voltage difference between the major anode and the major cathode exceeds the threshold voltage of the diode string, the diode string is turned on and effectively conducts current to quickly release voltage stress or trigger another circuit.

The newly-defined symbol in the right portion of FIG. 1 b will be used later to show in circuits possible locations for the diode string according to the invention.

FIG. 2 shows a circuit with electrostatic discharge (ESD) protection circuits according to the present invention. Diode strings can function alone or cooperate with other ESD protection devices to provide further protection. FIG. 2 exemplifies, but does not limit, the locations where diode strings are positioned. Diode string S1 is between a power line VH1 and an input/output (I/O) pad and diode string S2 is between a power line VL1 and the input/output (I/O) pad, both protecting circuitry connected to the I/O pad from ESD damage. Each of the diode strings S3-S6 is connected between two power lines to clamp the voltage difference therebetween, thereby protecting circuitry coupled across two power lines from ESD damage.

During normal operation, if the voltage supplied to the power line VH1 exceeds that supplied to the power line VH0, and the voltage supplied to the power line VL1 is lower than that supplied to the power line VL0, diode strings S1-S6 are closed. As a result, diode string S4 is reverse biased while diode strings S1-S3 and S5-S6 are forward biased during normal operation. Based upon the supply voltage arrangement, power line VH1 can be the same as power line VCC.

A diode string can function with an ESD protection trigger circuit in an ESD protection circuit. FIGS. 3 a and 3 b show two ESD protection circuits, each having an ESD protection trigger circuit according to the invention and connected between power lines VCC and GND. As shown in FIGS. 3 a and 3 b, diode string S7 connects with resistor R0 to form an ESD protection trigger circuit connected between power lines VCC and GND. The connection node between diode string S7 and resistor R0 can be connected to a trigger node of a primary ESD protection device. The primary ESD protection device is not limited to the bipolar junction transistor (BJT) B0 shown in FIG. 3 a, or a field effect transistor such as the metal-oxide-semiconductor transistor (MOS) M0 shown in FIG. 3 b. In FIG. 3 a, the trigger node is the base of BJT B0, and, in FIG. 3 b, the trigger node is the gate of MOS M0. When a pulse relatively positive to power line GND occurs on power line VCC, the voltage at the trigger node temporarily rises to trigger the primary ESD protection device and release ESD stress.

Depending on process technology and layout arrangement, a diode string according to the present invention may differ from the embodiment shown in FIG. 1 a. FIGS. 4 to 6 show cross sections of three diode strings further embodying the present invention.

The diode string in FIG. 4 can be fabricated by triple-well CMOS process. Unlike to the single deep N-well 112 in FIG. 1 a, there are several deep N-wells 112 together with N-wells 110. N-wells 110 and deep N-wells 112 have substantially the same voltage since they electrically couple to each other through interconnect and heavily-doped N regions 116.

The diode string in FIG. 5 can be fabricated by bipolarity complimentary metal oxide semiconductor (BICMOS) process that provides shallow and deep trench isolation. In FIG. 5, heavily-doped N sinkers 120 and a heavily-doped N buried layer 120 together become a separation region to isolate P-wells 106. Heavily-doped N sinkers 120 in FIG. 5 can be interchanged with normal N-wells. Around the separation region has a deep trench 124 and a shallow trench 126, as shown in FIG. 5, to separate N buried layer 120 from other N buried layers. Deep trench 124 and shallow trench 126 are made of isolating material.

Unlike FIG. 5, the diode string in FIG. 6 has several heavily-doped N buried layers 120 coupled to each other through interconnection and heavily-doped N regions 116, providing substantially equal voltage.

The power line connecting to resistor Rb in the embodiments need not have the highest voltage in a chip, but is required to have a voltage not less than that on the major anode.

While embodiments shown comprise a p substrate, the disclosure is not limited thereto. Those skilled in the art, after comprehending the above embodiments, can easily further derive embodiments with an N substrate and with changes in power supply accordingly. Embodiments with an N substrate are therefore omitted here for lucidity.

While the invention has been described by way of examples and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto, nor is the invention is limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1. A diode string characterized by low current leakage, comprising: diode regions providing diodes, each diode region comprising: a first polarity region of a first conductivity type, positioned on a substrate and acting as a first electrode of a corresponding diode; and a second polarity region of a second conductivity type opposite to the first conductivity type, positioned in the first polarity region and acting as a second electrode of the corresponding diode; wherein the diodes are forward connected in series to provide a major anode and a major cathode; a separation region of the second conductivity type isolating the diode regions from each other; and a bias resistor connected between the separation region and a first power line; wherein, during normal operation, the voltage of the first power line is not within the range between the voltages of the major anode and cathode.
 2. The diode string as claimed in claim 1, wherein the first conductivity type is P type and the second conductivity type is N type.
 3. The diode string as claimed in claim 2, wherein during normal operation, the voltage of the major anode does not exceed the voltage of the first power line.
 4. The diode string as claimed in claim 1, wherein the diode string comprises separation regions coupled to the first power line through the bias resistor.
 5. The diode string as claimed in claim 1, wherein the separation region comprises a deep well and a first well surrounding the diode regions, and the deep well is deeper than the first well.
 6. The diode string as claimed in claim 1, wherein the separation region comprises a buried region and a sinker region surrounding the diode regions, and the buried region and the sinker region are fabricated by BICMOS technology.
 7. The diode string as claimed in claim 1, wherein surrounding the separation region has a deep trench and a shallow trench made of isolating material.
 8. An electrostatic discharge (ESD) protection circuit on a chip, characterized by low current leakage, comprising: the diode string as claimed in claim 1; and second and third power lines, respectively coupled to the major anode and cathode of the diode string; wherein during normal operation, the diode string is forward biased and closed.
 9. The ESD protection circuit as claimed in claim 8, wherein the first power line and the second power line are the same power line.
 10. The ESD protection circuit as claimed in claim 8, further comprising: a trigger resistor, connected in series with the diode string and coupled between the second and third power lines; and a primary ESD device coupled between the second and third power lines, comprising a trigger node connected to the trigger resistor and the diode string.
 11. The ESD protection circuit as claimed in claim 10, wherein the primary ESD device is a BJT and the trigger node is the base of the BJT.
 12. The ESD protection circuit as claimed in claim 10, wherein the primary ESD device is a field effect transistor and the trigger node is the gate of the field effect transistor.
 13. A diode string characterized by low current leakage, formed, comprising: bipolar junction transistors (BJTs), each comprising a collector, an emitter and a base, wherein the emitter is connected to the base of a following BJT, and the base of the first BJT and the emitter of the last BJT act as two major electrodes of the diode string; and a bias resistor connected between the collectors of all the BJTs and a first power line; wherein, during normal operation, the voltage of the first power line is not within the range between the voltages of the major electrodes.
 14. The diode string as claimed in claim 13, wherein all the BJTs are NPN.
 15. The diode string as claimed in claim 14, wherein during the normal operation the voltage of the first power line is the highest voltage on a chip.
 16. An ESD protection circuit on a chip, characterized by low current leakage, comprising: the diode string as claimed in claim 13; second and third power lines, respectively coupled to the major electrodes of the diode string; wherein during the normal operation the diode string is forward biased and closed.
 17. The ESD protection circuit as claimed in claim 16, further comprising: a trigger resistor connected in series with the diode string and coupled between the second the third power lines; and a primary ESD device coupled between the second and third power lines and having a trigger node connected to the trigger resistor and the diode string. 